Apple Final Cut Express 4 - Manual (Page 721)

Chapter 46 Color Correction 721
IX
1 Move the Apply Above slider to the left to gradually include more of the brightest
areas of the picture in the range of what is to be desaturated.
2 Move the Softness slider to the right to ease the transition between the areas of the
picture being desaturated and the areas that are unaffected.
Depending on the range of values you’re choosing to desaturate, raising the softness
can keep this effect from looking artificial.
Move the Apply Above
slider to the left
to include more
bright areas.
Move the Softness slider
to the right to ease the
transition.
You can see the effects of
the filter in the Canvas.